A multiple input multiple output (MIMO) method using multiple antennas for a transmitter and a receiver is a technology that has received much attention in wireless/mobile systems, since MIMO performance is excellent due to its improved frequency efficiency and its diversity of transmission/receipt.
Singular value decomposition (SVD) is an example of a conventional MIMO method. SVD is a method in which a transmitter uses singular value decomposition of the channel matrix that is fed back from a receiver, thereby achieving maximum performance. However, generally, in a system such as frequency division duplexing (FDD), a receiver needs to notify a transmitter of information regarding a channel matrix so that a transmitter can know the channel matrix. In this regard, since the amount of information is very large, it is difficult to use the information in an environment where a channel varies according to a period of time.
In order to overcome this problem, research has been conducted on technologies in which the performance of a transmitter can be increased by feeding back partial channel information to the transmitter. Examples of such technologies include per antenna rate control (PARC) technologies, per stream rate control (PSRC) technologies, per unitary basis stream user rate control (PU2RC) technologies, etc.
In PARC technologies, an open loop channel capacity of a MIMO channel can be obtained theoretically by feeding back only signal-to-interference-plus-noise ratio (SINR) information for each antenna while using successive interference cancellation in a receiver.
In PARC technologies, data streams are transmitted to respective antennas. On the other hand, in PSRC technologies, data streams are precoded using a unitary matrix that is fed back from a receiving terminal so as to be transmitted. In addition, the receiving terminal feeds back a unitary matrix selected from among a plurality of unitary matrixes which can be used to precode data streams in a transmitting terminal, and then the data streams are precoded to respective column vectors of the unitary matrix so as to be transmitted. The receiving terminal also feeds back an SINR for each data stream to be precoded, and the transmitting terminal determines a data transmission rate of each stream by using the fed back SINR and transmits data.
Unlike in the case of PARC and PSRC technologies improving performance between transmitting and receiving terminals using multiple antennas, PU2RC technologies improve performance by using multi-user diversity when there are a plurality of terminals using multiple antennas. In PU2RC technologies, a base station transmits a plurality of data streams. Prior to transmitting the data streams, the data streams are precoded by respective column vectors of a unitary matrix. This method is the same as PSRC technologies except that respective data streams are transmitted to different users. The SINR for each data stream is fed back from respective users. A stream is assigned to a user having an optimum SINR from among the data streams so as to improve the performance of a MIMO system.
PU2RC technologies are advantageous compared to PARC and PSRC technologies in that performance can be increased using multi-user diversity in a multi-user environment. However, PU2RC technologies are disadvantageous in that interference between data streams cannot be cancelled by using successive interference cancellation unlike in PARC or PSRC technologies.
On the other hand, PARC and PSRC technologies are advantageous compared to PU2RC technologies in that interference between data streams can be cancelled by using successive interference cancellation. However, PARC and PSRC technologies are disadvantageous in that performance cannot be increased using multi-user diversity in a multi-user environment.
Actually, comparing PU2RC technologies with PARC or PSRC technologies, performances are different according to the number of users and channel environment in a system. Generally, as the number of users is increased, and the channel environment is closed to a line of sight (LOS), the performance of PU2RC technologies is better. On the other hand, as the number of users is reduced and the channel environment is closed to a rich scattering environment, PARC or PSRC technologies are better.
In successive interference cancellation based per user stream rate control (S-PUSRC) technologies that have been newly suggested as technologies simultaneously overcoming disadvantages of PARC, PSRC and PU2RC technologies, a receiver always needs to feed back decoding order used in successive interference cancellation to a transmitter. S-PUSRC technologies are disadvantageous compared is to PARC or PU2RC technologies in that feedback load is increased.